Scanning units are used for scanning a measuring standard, which, for example, may be implemented in the form of a scale having a coded or graduation track formed by a graduated scale, e.g., in the form of an incremental track, and, in addition to the coded track, a reference mark system. The scanning unit and the measuring standard are mounted, respectively, on one of two assemblies, which are movable relatively to one another and whose relative motion is to be measured. These may be, for example, a sliding carriage and the corresponding bed of a machine tool.
By scanning a coded track in the form of an incremental track composed of a periodic line graduation, the magnitude of the relative motion of the two assemblies with respect to one another can be ascertained. However, it is not possible to obtain any absolute positional information over great lengths. For that reason, to relate the relative motion of the two assemblies to a defined reference point, on one measuring standard, at least one reference mark is typically provided on which is used to identify a specific location on the measuring standard.
To scan a measuring standard of this kind, the scanning unit includes a first detector system or sensing array, for scanning the (incremental) coded track, as well as an additional detector system for scanning the reference track. In this context, however, the problem can arise, particularly when working with measuring standards constituted of a very long scale, that, during operation of the measuring device, it is, to a certain degree, not possible to bring the scanning unit into a position in relation to the associated measuring standard, where the scanning unit is able to sense the position of the reference mark of the measuring standard, for example, because of the danger of a collision with machine parts. For that reason, in the course of further development of a reference track constituted of one single reference mark, reference mark systems having coded reference marks have been developed. Reference mark systems of this kind have a multiplicity of reference marks arranged in series in the measuring direction (direction of extension of the incremental coded track) that are each uniquely differentiable from one another. Accordingly, each of these reference marks uniquely identifies a specific location on the measuring standard, so that multiple reference points are available as reference points for the position of the scanning unit in relation to the measuring standard. In this context, the reference marks can be coded in such a manner, for example, that they are disposed one behind the other or in series, spaced apart at different distances or spatial intervals. Then, in a process in which the scanning unit travels over two of the reference marks, a suitable algorithm is used to obtain the active (absolute) position of the scanning unit in relation to the measuring standard.
Conventional measuring devices have reference tracks including both simple as well as coded reference marks, the user being able to freely choose whether he/she will use the simple or the coded reference marks during operation of the measuring device. In this case, the detector system of the scanning unit assigned to the reference track has two sensors, one of which is assigned to the uncoded and the other to the coded reference marks. During operation of the measuring device, the one or the other sensor is optionally connected to an electrical amplifier which is supplied with the signals received by the sensor in question, as electric signals, during scanning of the measuring standard.
With regard to further details pertaining to the design of position-measuring systems or position encoders, which include a measuring standard and an associated scanning unit, reference is made to the textbook Digitale Laengen-und Winkelmesstechnik (Digital Linear and Angular Position Metrology) by Alfons Ernst, Publishers Moderne Indistrie (Modern Industry) (1998), pp. 9 through 38.
In the above-described measuring devices, the problem may arise that electrical interference, which acts, for example, on the sensor or on connecting lines between the sensor and the downstream electrical amplifier, can be misinterpreted as reference pulses by an evaluation unit (such as a machine control) downstream from the amplifier. This may lead to errors in the control of the machine tool in question and, in particular, may result in machine stoppage. To suppress interference, the amplifiers may be designed with low-pass filters. However, this noticeably retards the amplifiers and, thus, reduces the permissible traversing speed of the measuring device in question.